In recent decades, there has been an on-going effort to reduce the weight of vehicles in order to both save on material costs and to improve fuel economy. To decrease weight, vehicles have increasingly been designed to employ ever thinner metal sheets for exterior panel construction. However, an undesired consequence of such design changes has been poorer acoustic performance, which results in noticeably higher driving noise levels inside the car. Noise (vibration) may be generated, for example, by the engine, transmission or other mechanical moving parts of the vehicle. The acoustic vibrations generated by these sources are propagated throughout the vehicle so that they become audible or otherwise sensed by the occupants. These sound transmissions can be particularly caused by cavities or hollow spaces within the vehicle, such as the interior of a door, which can act as resonators. One solution to such problems has been to install sound-damping bulkheads or “pillar fillers,” which typically are polymer-based compositions that start foaming when exposed to heat (such as when the paint applied to a vehicle is cured in an oven) and fill and seal cavities in order to prevent or at least dampen the vibrations from being transmitted. Sound-deadening patches comprised of polymeric materials which are applied to panel surfaces have also been employed.
Considerable effort has been devoted to improving the vibration damping properties of such expandable polymer-based compositions. For example, Henkel AG & Co. KGaA has introduced Terophon® HDF (high damping foam), which is capable of damping noise-producing vibrations extremely fast and efficiently. However, the high performance damping materials available at present tend to have such highly effective performance only within a relatively narrow temperature range. The specifications set by vehicle manufacturers typically require peak damping performance at or somewhat above room temperature (20-25° C.), and known materials showing good performance at room temperature have reduced performance at temperature extremes. In particular, at cold temperatures such as those encountered during winter months in many climates (−20 to −10° C.), the high performance damping materials generally become too stiff and exhibit very little intrinsic damping.
For example, a high damping foam becomes stiffer (exhibiting a higher E′ value) as temperature decreases. The E′ value (Young's storage modulus, which is measured by Dynamic Mechanical Analysis) is defined as the ratio of tensile stress to tensile strain below the proportional limit of a material. As the stiffness of the foam increases, it has the potential to change the boundary conditions of a specific application, such as when the foam is being used to dampen vibration within a vehicle door. This change in boundary conditions may create a localized area of high vibration, which could reduce the overall acoustic performance of the application. In the case of a particular door assembly studied by the inventors, as E′ increased due to a lowering of the temperature of the door assembly containing a high damping foam, the vibration pattern on the door was found to change from three large displacement modes at 20° C. to six smaller displacement modes at −20° C.
Consequently, there is a need to develop ways of effectively damping vibrations within hollow structures such as vehicle doors over a broader temperature range than has previously been feasible.